The invention generally relates to methods for fabricating integrated circuits (ICs) and semiconductor devices and the resulting structures. More particularly, the invention relates to metal oxide semiconductor-field-effect transistor (MOSFET) devices and methods for making such devices. Even more particularly, the invention relates to improvements that may be made to trench-gate laterally-diffused MOSFET devices and methods for making such improved devices.
In IC fabrication, devices such as transistors may be formed on a semiconductor wafer or substrate, which is typically made of silicon. MOSFET devices are widely used in numerous electronic apparatus, including automotive electronics, disk drives and power supplies. Generally, these apparatus function as switches and are used to connect a power supply to a load.
One of the applications in which MOSFET devices have been used is for radio frequency (RF) applications. Such RF MOSFET devices are generally lateral transistors. See, for example, the lateral MOSFET device described in U.S. Pat. No. 5,949,104, as well as the device illustrate in FIG. 1. Such lateral MOSFET devices often have a diffused source that allows a backside contact for improved thermal conducting and reduced parasitics.
Recent advances in lateral (or laterally-diffused) MOSFET (LDMOS) devices have improved the performance and cost characteristics of lateral MOSFET devices when compared to vertical MOSFET devices for RF power amplifiers in base stations applications. Such RF LDMOS devices have been particularly useful for wireless base station applications. The RF vertical (or vertically-diffused) VDMOS structure unfortunately suffers from certain limitations relative to the LDMOS such as high output capacitance (which decreases efficiency), decreased power gain, narrowing of the usable bandwidth, and source inductance that decreases the operating efficiency.
Thus, what is needed are circuits, methods, and apparatus that provide an improved LDMOS having reduced output capacitance, increased power gain, and more useable bandwidth.